Heretofore, for a developer applicable to the image formation in an electrophotographic system, two kinds of roughly classified developers have been used, namely, a single-component type developer and a two-component type developer. Particularly among color developers, a two-component type developer has been advantageous from the point that a stable charging property can be secured.
As the multicolor image forming processes in which the two-component type developer is used, there have been two known developing processes, namely, a contact developing process characterized in making use of an intermediate image transfer member and a non-contact developing process characterized in putting colors on a latent image carrier one over another without making use of any intermediate image transfer member. From the viewpoint that the process itself can be miniaturized, the non-contact developing process has been advantageous.
However, in a simple non-contact developing process, a developability is usually deteriorated as compared to the contact developing process. JP OPI Publication No. 3-271753/1991 and so forth, therefore, overcome the above-mentioned problem in such a manner that a developer layer is made thinner to bring a distance between a latent image carrier member and a developer carrier member (developing roller) closer so that the electric field of a development can be intensified.
As a means for achieving an extreme thin developer layer, there are the following known means; namely,
Item 1. a means for pressing a developer layer regulating rod against the surface of a developing roller;
Item 2. another means for regulating a developer layer by bringing an elastic blade into contact with the surface of a developing roller; and
Item 3. a further means for regulating a developer layer by keeping a specific gap between a non-elastic blade and the surface of a developing roller.
Among the means, the following means are effective to form a layer having a stable layer thickness, namely; a means for pressing a developer regulating rod against the surface of a developing roller, mentioned in item 1; and a thin layer forming process carried out by making use of a rigid rod-type magnetic member, that is proposed in JP OPI Publication No. 2-50184/1990. However, when making use of the above-given processes, there is such a disadvantage that a developer used therein is received by an excessive stress, though there is such an advantage that a stable layer can be formed. Particularly when miniaturizing an equipment, it is expected that the state of things may getting more serious. The increase of the above-mentioned stress in forming a thin layer may cause the destroy or peeling-off of a carrier-coated layer and may also seriously affect the durability of a developer used.
With the advance of the miniaturization of an equipment and a developing apparatus, it becomes an important thema for a two-component type developer how to electrically charge rapidly and properly within a period between a time when a toner is supplied and a time when the toner is transferred to a development nip section. Heretofore, it has been usual to add a negatively chargeable charge-control agent to a toner so as to improve the charge-rising property of a negatively chargeable developer. However, when making use of such a miniaturized developing apparatus as mentioned above, only the addition of a charge-control agent is not enough, because a toner is scattered in the developing apparatus and an image is also seriously fogged by the increase of the amount of a weakly charged toner. As a means for improving an electric charge rising property on a carrier side, a positive charge control agent is added to a carrier, such as described in JP OPI Publication No. 2-8860/1990. The positive charge controlled agents include, for example, a quaternary ammonium compound such as those disclosed in JP OPI Publication No. 52-10141/1977, and an alkyl pyridinium compound and an alkyl picolinium compound (including, for example, nigrosine SO and nigrosine EX) such as those disclosed in JP OPI Publication Nos. 56-11461/1981 and 54-158932/1979. These charge control agents an organic compound having a high cohesive property and, accordingly, they have a poor dispersibility. It has, therefore, been liable to produce a charge failure with toner, because a charge control agent is maldistributed or extricated in the coated layer of a carrier. When a toner component is fused to a carrier, that is, when producing a so-called spent in making a multicopying, the charge rising property cannot be stabilized in making the multicopying, because a charge control agent component made present on the surface of the carrier is covered by the toner component.
As described in JP OPI Publication Nos. 57-168256/1982, 59-228261/1984, 63-71860/1988 and 2-110577/1990, the attempts for improving the environmental differences of chargeability between developers have been tried to inhibit the variation of a water-absorption by covering a magnetic particle with a silicon resin or by adding an inorganic fine particle subjected to a hydrophobic treatment to a coated layer.
However, even in the above-mentioned attempts, the hydrophobic treatment cannot be enough for allowing to stand under the conditions of a high temperature and a high humidity for a long time, but a variation of the charging function of a carrier is observed and, there still remains such a problem that a developability is varied by the variation of the amount of a developer transported, that is produced in a thin-layer forming section by the variation of the above-mentioned charging function of the carrier.
As the means for preventing a chargeability variation produced by a toner-spent, it has been carried out the addition of silica with the purpose of abrading a spent toner, as described in JP OPI Publication Nos. 54-21730/1979, 58-117555/1983 and 59-232362/1984. However, silica applied thereto has a few abrading effect, because it is usually the spherical form. Further, in the case of such a system having a great stress as in a thin-layer forming process, silica has such a defect that it is split off. Therefore, the abrading effect of the silica cannot be kept on, though the spent production may be relatively retarded as compared to a carrier without adding silica thereto, and silica is completely split off after making a multicopying and, thereby, a lot of the spent are produced. Therefore, a charged amount is seriously varied so that a toner flying and a background fog are resultingly induced.